Implementing-cryptography-using-python

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CHAPTER9Mastering CryptographyUsing PythonThe time has come to put all you have learned into action; you should now understandthe concepts you need to secure communications between two parties.The chapter will focus on building an application that can send messages inplaintext, or encrypted using asymmetric public key infrastructure (PKI) andusing the symmetric key method with an Elliptic Curve Diffie-Hellman keyexchange. Implementing these techniques between two applications over aninsecure UDP connection will help you think through how to send messagessecurely and ensure that they have not been tampered with. We will use Wireshark,a popular network analyzer, to verify that the messages are encryptedand cannot be distinguished from random noise. We will highlight using AES(Advanced Encryption Standard) in Counter (CTR) and Galouis/Counter (GCM)modes. We covered CTR in Chapter 5, and while the implementation of GCM isquite similar, it requires some special consideration as we design our encryptionprotocol. Our first task will be to build a small working application for plaintextcommunications, then adding the other cryptographic components to the mix.Throughout this chapter, you gain cryptographic experience as you:■■Construct an application that communicates in plaintext■■Install Wireshark and examine the communication traffic■■Implement a PKI into the application■■Implement RSA Digital CertificatesImplementing Cryptography Using Python®, First Edition. Shannon W. Bray.© 2020 John Wiley & Sons, Inc. Published 2020 by John Wiley & Sons, Inc.247
248 Chapter 9 ■ Mastering Cryptography Using Python■■Encrypt the message using ECC■■Implement the Elliptic Curve Diffie-Hellman exchangeConstructing a Plaintext Communications ApplicationOur first task is to revisit the construction of a threaded server with unencryptedtraffic, which was introduced in Chapter 7. In addition to building a server andclient Python file, we will create a helper file here that will support the codingadditions we will implement as we are building our Python solution.The architecture for the application includes a server file (crypto _ server _ a.py), a client file (crypto _ client _ a.py), and a helper file that will be used byboth the client and server since they share many of the same components. Oneside encrypts, and the other one decrypts. For the sake of keeping this examplesimple, we will only do one-way communication. The same techniques used inthis chapter can be used between any applications that transmit data over UDP.Creating a ServerOur first step is to create an application that will use UDP sockets to acceptpackets from other programs. For our example we can use the localhost so thatyou can use the client application to send plaintext messages to the server. Theserver will then display the text as received.Here’s the Python code:# Message Receiver - crypto_chat_server.pyimport hashlib, random, os, timefrom binascii import hexlifyfrom socket import *import Chapter9.ch9_crypto_chat as ctdef get_dh_sharedsecret():returndef get_dh_sharedkey():returndef decrypt(ciphertext, usePKI, useDH, serverSecret):#msg = ct.decrypt(ciphertext, usePKI, useDH, serverSecret)try:msg = ct.decrypt(ciphertext, usePKI, useDH, serverSecret)except:msg = ciphertextreturn msg
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ImplementingCryptography UsingPytho
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To Stephanie, Eden, Hayden, and Ken
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viAbout the Authorbook, Shannon is
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Contents at a GlanceIntroductionxvi
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xiiContentsUsing Conditionals 16Usi
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xivContentsPseudorandomness115Break
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xviContentsInstalling and Testing W
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xviiiIntroductionprivacy advocates.
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CHAPTER1Introduction to Cryptograph
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Chapter 1 ■ Introduction to Crypt
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CHAPTER2Cryptographic Protocolsand
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Chapter 2 ■ Cryptographic Protoco
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66 Chapter 3 ■ Classical Cryptogr
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CHAPTER4Cryptographic Mathand Frequ
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Chapter 4 ■ Cryptographic Math an
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CHAPTER5Stream Ciphers and Block Ci
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Chapter 5 ■ Stream Ciphers and Bl
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CHAPTER6Using Cryptography with Ima
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Chapter 6 ■ Using Cryptography wi
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Page 290 and 291: IndexSYMBOLS\ (backslash), 10- oper
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Page 294 and 295: Index ■ I-M 281hashlib module, 28
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